Sack Lunch Seminar (SLS)

Stability and Internal Flow Variability of Ice Sheets

Ice streams are regions of fast-flowing ice embedded within ice sheets that account for the majority of mass transport from ice sheet interiors to the ocean. Variability of ice stream flow on centennial to millennial time scales plays an important role in the present mass balance of the West Antarctic Ice Sheet. In this talk, I show how a simple model of subglacial meltwater production coupled to ice flow explains the underlying physical mechanism for millennial-scale, unforced ice stream variability and predicts the transition to steady ice stream flow. The model equally well reproduces modern ice stream variability in the Siple Coast region of West Antarctica and Heinrich events, periods of increased ice discharge from the Laurentide Ice Sheet during the last glacial period. In a more realistic, purpose-built model, the same mechanism produces variability and rapid migrations of the ice stream grounding line. These migrations are always associated with mass imbalance near the grounding line, but not necessarily in the ice stream at large, which is important to consider when interpreting modern observations of grounding line variability. Under certain conditions, this ice stream variability may cause the grounding line to slow down for hundreds to thousands of years even as it retreats onto a reverse bed slope, before readvancing. Such behavior runs counter to the conventional theories predicting the instability of ice sheets on reverse bed slopes. Determining if such behavior occurs in real ice sheets is important when evaluating the likelihood of irreversible ice sheet collapse and rapid sea level rise in the future.